1. Field of the Invention
The present invention relates to axial flow compressors used on such as gas-turbine engines or jet engines, and in particular, relates to a method for predicting inception of a rotating stall in these axial flow compressors.
2. Description of the Related Art
In general, efficiency of an axial flow compressor such as a gas turbine compressor or a jet engine increases as the pressure ratio increases and as air flow supplied into the compressor decreases. However, in the vicinity of the peak pressure ratio, an unstable phenomenon called a stall occurs and the compression efficiency remarkably decreases. Conventionally, in order to avoid occurrence of the stall, the axial flow compressors are operated under a condition, apart from the operating condition, which is likely to cause the stall. Accordingly, it was not possible to operate conventional axial compressors at relatively high efficiency. On the other hand, when an axial compressor enters into the fully developed stalled state, although it is possible to detect the stall occurrence by compressor discharge pressure drop or by noise from the compressor, a technology to predict the stall or prediction of the stall initiation is not realized yet.
The present invention was made to solve the above-described problems and the object of the present invention is to provide a method of predicting the stall occurrence of an axial flow compressor, including a method for reliably predicting the operating condition is in the vicinity of stall occurrence point, and a method for increasing the pressure ratio to a peak thereof by controlling the operating conditions of the axial flow compressor, and a method of improving the efficiency of the axial flow compressor.
In order to solve the subject of the present invention, the stall prediction method of the axial flow compressor by the present invention comprises a pressure sensor provided on a casing wall adjacent to a leading edge of the rotor blade thereof, and the stall prediction is performed by monitoring the time history data output by the aforementioned pressure sensor.
Since the air flow is least stable in the vicinity of the leading edge of the rotor blade, it is possible to find the smallest change of the air flow and to predict the stall occurrence in advance by providing the pressure sensor adjacent to the leading edge of the rotor blade.
According to one aspect of the present invention, it provides a pressure sensor adjacent to the leading edge of the rotor blade and the stall inception can be predicted by the time sequential data obtained by the above-described pressure sensor.
Since the air flow is not disturbed adjacent to the leading edge of the rotator blade till immediately before the stall arises, provision of the pressure sensor on the casing wall adjacent to the leading edge of the rotor blade makes it possible to reliably predict occurrence of the stall by monitoring the time history data output from the pressure sensor. Therefore, since it is possible to operate the compressor at conditions close to the peak efficiency, the efficiency can be dramatically increased.
In the present stall prediction method, the time history data output from the pressure sensor are divided into a plurality of data sets for respective unit cycle time and the stall prediction is performed by calculating the autocorrelation of one cycle data set comparing with a previous cycle data set or with a data set several cycles before. Thus, the transition process of the inter-blade flow to the stall cell can be detected.
According to one aspect of the present invention, plural sensors are provided in the axial direction of the rotor, so that it is possible to rapidly detect the imminent stall.
Since the time span to calculate the autocorrelation value is set to a data corresponding to one rotor blade, it is possible to rapidly calculate the autocorrelation.
According to the present invention, the stall prediction method detects the stall by monitoring a time history data output from the pressure sensor provided on the casing wall adjacent to the leading edge of the rotor blade facing the leading edge at twelve o""clock location.
The present invention is capable of predicting the stall by a sensor at a location where disturbance first appears. For example, when a gas turbine is started after being stopped for a certain period, sometimes the stall occurs if that period is comparatively short. This is because, when an engine stops, although the lower portion of the compressor duct cools rapidly, the upper portion of the compressor duct cools slowly, so that sometimes the clearance between the end tip of the rotor blade and the inner surface of the casing wall becomes larger, and this larger clearance sometimes causes the stall. When the pressure sensor is located at the upper portion of the duct where the stall is most liable to occur, it is possible to provide the fastest indication.
The stall prediction method of the present application makes it possible to predict the stall inception at the earliest time by the processing method of data provided from a high response pressure sensor placed on the casing wall at circumferential location where rotating stall is likely to start and axial location close to the leading edge of the rotator blade